Nitric acid (HNO3) is a colorless, liquid acid widely used in the manufacturing of explosives and fertilizers. When dissolved in water, molecules of nitric acid separate (or dissociate) into hydrogen ions (H+) and nitrate ions (NO3). The fact that nearly every nitric acid molecule dissociates is what makes nitric acid a strong acid. Nitric acid is often the starting material in the industrial production of nitrates for fertilizers.
Plants take up nitrogen from the soil in the form of ammonium ions (NH + 4 ) and nitrate ions, and along with carbon containing molecules made during photosynthesis, these ions are used to synthesize amino acids, from which proteins are made. Within the past hundred years the demand for nitrogen fertilizers has grown dramatically as the need for fertilizers for agriculture has grown. The natural manner in which nitrates reach the soil involves the reaction of nitrogen gas and oxygen gas in the atmosphere to form nitrogen dioxide gas (NO2), which then reacts with atmospheric water, making nitric acid, which provides a natural source of nitrates in water and soil.
During World War I, the Germans were very interested in using ammonium nitrate (NH4NO3), a salt of nitric acid, as an explosive. Many organic nitrates such as nitroglycerine and TNT are also highly explosive. Nitric acid is made by the reaction of ammonia with oxygen gas. The nitric acid which is produced can then be used to make a variety of compounds. This is a process that has allowed large amounts of fertilizers to be produced relatively inexpensively.
Nitric acid is also formed from the reaction of nitrogen oxides produced during the combustion (burning) of fossil fuels in automobile engines. These nitrogen oxides react with water in the atmosphere and form nitric acid, one cause of acid rain. High levels of nitrates in drinking water can contribute to the formation of nitrosamines, a group of carcinogenic (cancer causing) compounds.